detect improper use of t.throws

Protects against a common misuse of t.throws (Like that seen in #739).

This required the creation of a custom babel plugin.

https://github.com/jamestalmage/babel-plugin-ava-throws-helper

lib/caching-precompiler.js

@@ -52,11 +52,13 @@ CachingPrecompiler.prototype._init = function () {
 	];
 
 	var transformRuntime = require('babel-plugin-transform-runtime');
+	var throwsHelper = require('babel-plugin-ava-throws-helper');
 	var rewriteBabelPaths = this._createRewritePlugin();
 	var powerAssert = this._createEspowerPlugin();
 
 	this.defaultPlugins = [
 		powerAssert,
+		throwsHelper,
 		rewriteBabelPaths,
 		transformRuntime
 	];

lib/test.js

@@ -68,6 +68,24 @@ Test.prototype._assert = function (promise) {
 };
 
 Test.prototype._setAssertError = function (err) {
+	var data = err._avaThrowsHelperData;
+	if (data) {
+		console.error(
+			[
+				'Improper usage of t.throws detected at %s (%d:%d).',
+				'You should wrap the following expression in a function:',
+				'  %s',
+				'Like this:',
+				'  function() {\n    %s\n  }',
+				'See https://github.com/sindresorhus/ava#throwsfunctionpromise-error-message for more details.'
+			].join('\n\n'),
+			data.filename,
+			data.line,
+			data.column,
+			data.source,
+			data.source
+		);
+	}
 	if (this.assertError !== undefined) {
 		return;
 	}

package.json

@@ -79,6 +79,7 @@
     "arrify": "^1.0.0",
     "ava-init": "^0.1.0",
     "babel-core": "^6.3.21",
+    "babel-plugin-ava-throws-helper": "0.0.4",
     "babel-plugin-detective": "^1.0.2",
     "babel-plugin-espower": "^2.1.0",
     "babel-plugin-transform-runtime": "^6.3.13",

test/caching-precompiler.js

@@ -6,6 +6,7 @@ var uniqueTempDir = require('unique-temp-dir');
 var sinon = require('sinon');
 var babel = require('babel-core');
 var transformRuntime = require('babel-plugin-transform-runtime');
+var throwsHelper = require('babel-plugin-ava-throws-helper');
 var fromMapFileSource = require('convert-source-map').fromMapFileSource;
 
 var CachingPrecompiler = require('../lib/caching-precompiler');
@@ -145,7 +146,7 @@ test('uses babelConfig for babel options when babelConfig is an object', functio
 	t.true('inputSourceMap' in options);
 	t.false(options.babelrc);
 	t.same(options.presets, ['stage-2', 'es2015']);
-	t.same(options.plugins, [customPlugin, powerAssert, rewrite, transformRuntime]);
+	t.same(options.plugins, [customPlugin, powerAssert, throwsHelper, rewrite, transformRuntime]);
 	t.end();
 });
 

test/cli.js

@@ -96,6 +96,14 @@ test('throwing a named function will report the to the console', function (t) {
 	});
 });
 
+test('improper use of t.throws will be reported to the console', function (t) {
+	execCli('fixture/improper-t-throws.js', function (err, stdout, stderr) {
+		t.ok(err);
+		t.match(stderr, /Improper usage of t\.throws detected at .*improper-t-throws.js \(4:10\)/);
+		t.end();
+	});
+});
+
 test('babel require hook only applies to the test file', function (t) {
 	t.plan(3);
 

test/fixture/improper-t-throws.js

@@ -0,0 +1,9 @@
+import test from '../../';
+
+test(t => {
+	t.throws(throwSync());
+});
+
+function throwSync() {
+	throw new Error('should be detected');
+}